dry air humid humidity COVID

Dry vs Humid Air: Which Spreads COVID-19 More Readily?

MedicalResearch.com Interview with:
Dr. Ajit Ahlawat (en.
Scientific staff (Post-Doc), Department Experimental Aerosol & Cloud Microphysics
Leibniz Institute for Tropospheric Research (TROPOS)

MedicalResearch.com: What is the background for this study?  

Response: World Health Organization (WHO) has recently acknowledged that novel coronavirus (COVID-19) can be transmitted via aerosols after an appeal from 239 scientists from 32 countries including the chemist Prof. Hartmut Herrmann from our institute i.e. TROPOS. In order to contain the spread via the aerosol particles floating in the air, the researchers recommend not only continuing to wear masks but also, and above all, good indoor ventilation.

In aerosol research, it has been long known that air humidity plays a major role i.e. either at high humidity, more water adheres to the particles and so they can grow faster or at low humidity, evaporation occurs and particle will lose its water content. So, we were curious about what types of studies have already been conducted on this topic and how indoor relative humidity (RH) will influence SARS-CoV-2 airborne transmission in indoor environments.

MedicalResearch.com: What are the main findings? 

Response: Here, we explore the role of relative humidity (RH) in airborne transmission of SARS-CoV-2 virus in indoor environments based on recent studies around the world.

Humidity affects both the evaporation kinematics and particle growth. Air humidity influences the spread of corona viruses indoors in three different ways:

(a) the behaviour of microorganisms within the virus droplets,
(b) the survival or inactivation of the virus on the surfaces, and
(c) the role of dry indoor air in the airborne transmission of viruses.

Although, low humidity causes the droplets containing viruses to dry out more quickly, the virus survivability remains still high. If the relative humidity of indoor air is below 40 percent, the viral particles emitted by infected people starts evaporating and fly further through the room and are more likely to be inhaled by healthy people. In addition, dry air also makes the mucous membranes in our noses dry and more permeable to viruses. In dry indoor places i.e., less humidity (< 40% RH), the chances of airborne transmission of SARS-CoV-2 are higher than that of humid places (i.e., > 90% RH). Based on earlier studies, a relative humidity of 40–60% was found to be optimal for human health in indoor places.

MedicalResearch.com: What should readers take away from your report? 

Response: Firstly, an important information on indoor relative humidity may be a practical way to make general public aware regarding its role in SARS-CoV-2 transmission. From a researchers’ point of view, more attention should be paid to indoor air in order to prevent future outbreaks of viral disease.

Secondly, policymakers can deploy a number of measures for controlling the indoor humidity. For countries in colder climates, minimum RH standard for the indoor environments should be kept into consideration avoiding extreme heating indoors. For countries in tropical and hot climates, humidity control measures are recommended while avoiding extreme cooling of indoor places.

Lastly, the government officials should set some regulations worldwide that require a minimum relative humidity standard in public buildings and indoor environments. Based on research findings, for future scenarios, setting a minimum RH standard of 40% for public buildings will not only reduce the impact of COVID-19, but it will also reduce the impact of further viral outbreaks, both seasonal and novel.

MedicalResearch.com: What recommendations do you have for future research as a result of this work?

Response: Researchers should examine the relative importance of relative humidity in airborne transmission of SARS-CoV-2 in indoor microenvironments using novel sensor-based devices along with routinely used instruments.

They should conduct large number of experimental studies regarding role of relative humidity in viral spread in public buildings like hospitals, buses and offices etc. This will help in collection of large datasets and thereafter with the help of modeling, one can easily predict how exactly indoor RH affects the SARS-CoV-2 and future viral outbreaks. 

MedicalResearch.com: Is there anything else you would like to add? 

Response:  In order to implement the above mentioned guidelines, we need a concrete plan along with the relationship between different communities such as medical professionals, policy makers, planners and government officials. In order to curb the disease outbreaks, we must focus on the role of indoor air on disease transmission and resident health. Other precautions apart from RH optimization is to increase natural ventilations like opening of windows during indoor stay, using proper face masks (face shields along with face mask could provide better results), avoid staying in direct periphery of the infected or other persons, and maintaining social distancing. 

No disclosures


Ajit Ahlawat, Alfred Wiedensohler, Sumit Kumar Mishra. An Overview on the Role of Relative Humidity in Airborne Transmission of SARS-CoV-2 in Indoor Environments. Aerosol and Air Quality Research, 2020; 20 DOI: 10.4209/aaqr.2020.06.0302

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Last Updated on August 24, 2020 by Marie Benz MD FAAD